Fiber reinforced composites (FRC) are used increasingly in a wide range of applications due to the versatility they provide to the design process. By combining the properties of the constituent components, one can generate a unique set of requisite properties unattainable by the individual components alone. Among the most impressive applications of FRC are those associated with the use of (1) epoxy resins to produce laminates for aircraft. (2) cement prefabricated constructs for use in residential, industrial, and military buildings and bridges; and (3) acrylic resins in orthopedic and dental prostheses. All of these applications call for moderate to low density fibers with exceptional strength and stiffness, such as glass, carbon, aromatic polyamide (e.g., Kevlar.RTM.) and ultrahigh molecular weight polyethylene fibers.
It is also well recognized that the properties of a composite depend largely on the nature of the interface or boundary region formed between the reinforcing fibers and surrounding matrix. Consequently, it has become clear that it is critical to tailor the properties of the interface to achieve the desired properties in composites. Toward increasing the effectiveness of these fibers, there have been numerous efforts to modify the fiber surface to physicochemically integrate with the matrix, through unique interfaces which allow transferring the load from the matrix to the high strength fibers.
For glass fibers, among the most common approaches for surface modification is the use of silane coupling agents. Contemporary methods of glass surface modification entailed covalently binding polymeric chains onto the glass surface. In a novel approach to development of absorbable glass composites, polylactones have been grafted on surface-modified silicophosphate fibers. Having such a hybrid interface between an inorganic filler and organic matrix allows the development of a unique adhesive joint through chain mixing. Although many recent attempts have been made to develop unique interfaces between carbon or Kevlar.RTM. fibers and different matrices, traditional use of sizing agents of limited effectiveness still prevails.
Of all the available high modulus fiber reinforcing agents, ultra-high molecular weight polyethylene (UHMW-PE) has the most promise because of its lower density as compared with glass, carbon, and Kevlar.RTM. as well as having higher toughness than carbon and glass. However, a major drawback in UHMW-PE fibers is their hydrophobic surface and incompatibility with most organic matrices that are used in fiber-reinforced composites. To address the hydrophobicity problem, many attempts have been made to modify the UHMW-PE surface by (1) introducing amine functionalities using ammonia plasma; (2) forming hydroxylic groups by oxygen plasma or chromic etching; (3) calendaring and surface oxidation; and (4) surface phosphonylation and subsequent hydrolysis of the phosphonyl dichloride moieties. The plasma-oxidized fibers showed some improvement when used in epoxy FRC, as compared to untreated fibers.
The use of the relatively low density, ultrahigh strength, high modulus fibers in the development of high performance, fiber-reinforced composites has been viewed by many as an excellent means to produce novel materials for use in traditional industrial applications as well as novel biomedical prostheses. However, the development of such composites has been compromised by the incompatibility of the UHMW-PE surface with useful matrices and, hence, poor wetting of the fibers. This is associated with poor fiber/matrix interfaces which led to poor ability for load transfer from matrix to fibers. The low density of UHMW-PE fibers, also limited their use at the traditional high fiber loading (40%-60%). Such a situation created a need for a novel approach for producing high performance FRC using modified forms of UHMW-PE at low and exceptionally low fiber loading.
The significance of having chemically continuous or tailored interfaces to physicochemically abridge the fibers to matrix has been demonstrated earlier. Thus, a novel form of self-reinforced UHMW-PE was developed and shown to exhibit exceptional mechanical properties at fiber loading of less than 10%.